Devil Bringer

The Devil Bringer is Nero's right arm, and the physical manifestation of his demonic power. Although it has not been explicitly explained how Nero received the Devil Bringer, there is an Order of the Sword report that indicates that Nero received an injury to his shoulder after protecting Kyrie from a demon attack within the Mitis Forest, before the events of the game. According to Nero, "From that day forth, [his] arm changed and a voice echoed 'Power. Give me more power.'"

The arm possesses superhuman strength, which it manifests telekinetically through a larger, spectral arm. It is capable of absorbing magical items, such as the Yamato or Evil Legacy, and manifesting their power. Although Nero doesn't often use his demonic arm directly, he does wield his Blue Rose with it, allowing him to channel demonic energy into the gun.

History
The story of Devil May Cry 4 focuses a large majority of the plot on the Devil Bringer, its powers, and its origins. At first, the arm disturbs his lover, Kyrie, due to its obviously demonic origin, but after Nero uses it to save her from Sanctus, she comes to accept Nero, no matter what.

Upgrade Items
The Devil Bringer is able to absorb certain Key Items and use their power to grant itself new abilities. These items are obtained either by finding them within special white pillars which project colored beams, or by taking them from specific bosses.

Evil Legacy
The Evil Legacy (負の遺産) is found within a blue pillar in the Cathedral. It is a special crystal that holds the power of an ancient evil, and after being absorbed into the Devil Bringer it grants Nero the ability to perform Snatch by locking onto small enemies, or Hell Bound by locking onto Grim Grips and large enemies. Many Grim Grips require Nero to step upon a blue Continuum Pad to become activated, though there are more that do not. Once obtained, Snatch can be used during any mission, including those "before" it was found.

Anima Mercury
The Anima Mercury (水銀のアニマ) is found within a yellow pillar in the Library. It is an artificial soul made through alchemy that can be used to animate Gyro Blades. Nero can move a Gyro Blade by using Buster near it, and can speed it up by striking it with his sword before moving it with another Buster. Animated Gyro Blades can be used to destroy barriers and objects or shred enemies.

Rusalka Corpse
The Rusalka Corpse (ルサルカの亡骸) is one of Bael's feelers, obtained after Nero rips it off his head. After being absorbed by the Devil Bringer, it grants Nero the Auto-Search ability. When near hidden Red Orbs, Blue Orb Fragments, or Secret Missions, the Devil Bringer will begin glowing, strengthening as it gets nearer to the hidden object. Once obtained, Auto-Search can be used during any mission, including those "before" it was found.

Sephirothic Fruit
The Sephirothic Fruit (セフィロトの実) is one of Echidna's seeds, obtained after it falls from her as she flees. After being absorbed by the Devil Bringer, it grants Nero the ability to wither certain demonic trees that entangle areas of Mitis Forest and the Order of the Sword H.Q..

Aegis Shield
The Aegis Shield (アイギスの楯) is Angelo Credo's shield, obtained after Nero defeats him. After being absorbed by the Devil Bringer it grants Nero the ability to perform Hold. Once obtained, Hold can be used during any mission, including those "before" it was found.

Buster
Every enemy within Devil May Cry 4 can be damaged with Nero's Buster technique. This technique will grab the enemy and attack them in a method varying depending on the enemy. Lesser demons have different Busters depending on whether Nero is on the ground, in mid-air, or in Devil Trigger when he attacks, while Bosses can have several cinematic Buster moments which vary depending on the circumstances of both the boss and Nero. Some of these will not actually damage the boss, and will result with it using an attack on Nero. During most of these cinematic Busters, Nero can fire Summoned Swords at the boss, and for some of them he may use Exceed to increase the damage dealt.

Lesser Demons
Nero will grab the Scarecrow and slam them into the ground. If in the air, Nero will throw the Scarecrow to the ground in a similar fashion. A grounded Devil Buster will slam the Scarecrow into the ground three times, while an airborne Devil Buster will throw them at the ground so hard that they bounce back, allowing Nero to Buster them again.
 * Scarecrow (Arm) and Scarecrow (Leg)

It seems to be the "default" Buster move: on some other enemies executing Buster results in the same animation.

Nero grabs the Scarecrow and slams enemy into ground before he kicks it away. When performed in the air, Nero spins horizontally and hammer-tosses it, and stays aloft. When Devil Triggered, the Mega is slammed 3 times before being kicked, and is more violently spun while in the air.
 * Mega Scarecrow

Nero grabs the Frost's tail and swings them around rapidly before flinging them in any direction. When performed in the air, Nero stays aloft. When Devil Triggered, the spinning is more vicious. Pressing the Buster button while Nero's spinning it will increase the number of spins to a max of 3 for Buster and 5 for Devil Buster.
 * Frost

Nero grabs the Assault by the tail, slams it into the ground several times, and slams it down in front of him. When performed in the air, Nero will drag the Assault to the ground before doing the same. If Nero uses Buster on an Assault right before it's spinning aerial attack makes contact Nero will instead float in the air then violently slam it into the ground. Pressing the Buster button while Nero's slamming it will increase the number of slams to a max of 3.
 * Assault

Nero unleashes a series of punches and kicks to the Blitz before uppercutting it away. If done in the air, Nero unleashes a similar barrage before hammering the Blitz into the ground.
 * Blitz

Nero holds the Gladius by its sword-handle and throws it at another demon. Does minimal damage by itself, but if it hits another enemy will deal more damage to both. When Devil triggered, Nero spins around and tosses it like a spinning blade.
 * Gladius

Nero grabs the tail of the Cutlass and leaps into the air, spinning the fish rapidly before slamming it into the ground. When performed in the air, Nero returns to the ground for the finale of this buster.
 * Cutlass

Nero grabs the Basilisk, and forces it to shoot a fireball forwards. When performed in the air, the fireball is shot downwards instead. More shots are fired in Devil Trigger.
 * Basilisk

Nero tears the Chimera Seed in half, killing it instantly.
 * Chimera Seed

Nero will attack the Chimera as he would attack its normal host, though the Chimera Seed will continue to flail its tentacles.
 * Chimera

While the demon is cloaked, Nero's Buster will strip more of its cloak away than a Snatch, and the demon will retreat slightly. With its cloak broken, Nero will slam it into the ground similar to a Scarecrow. A Devil Buster will slam the demon into the ground three times.
 * Mephisto and Faust

A Buster from the front will damage the Angelo's shield, but not the Angelo itself. If used from behind, in the air, or when the shield is broken, performing a Buster will cause Nero to impale the Angelo with its own lance, rev it several times, then shoot it across the room. A Devil Buster will cause Nero to hold onto the Angelo as it is propelled by the lance, dealing more damage. Pressing the Buster button while Nero's revving the lance it will increase the number of revs to a max of 3.
 * Bianco Angelo

A Buster from the front will damage the Angelo's shield, but not the Angelo itself. Once the shield is broken, performing Buster will cause Nero to grab the Angelo and perform a suplex on it and then dropkick it in the chest. It is also possible to Buster the Alto Angelo during the instant it is stunned from a strong attack, though this is quite difficult unless a Chrono Slicer is activated. A Devil Buster will change this to three rolling suplexes followed by Nero pulling it into the air and then drilling it into the ground. Performing either Buster while airborne will cause Nero to drag the Angelo to the ground before performing the same. Also, when firing an energy ball with the Bianco Angelos, Nero can Buster it to send the ball back. This results into killing the Biancos instantly although the Alto may survive.
 * Alto Angelo

Adjudicator's Buster is equal to Scarecrow's, except that in Devil Trigger Nero will slam it two times instead of three.
 * Combat Adjudicator

Bosses
When Berial's flames are extinguished, Nero can either haul Berial up and slam him into the ground (mid-air Buster) or fling him up before dealing a damaging blow to the face. When Devil Triggered, he lifts him up and unleashes a quick barrage of slashes with the Yamato while the demon is in midair and then slams him to the ground. If on the ground while Devil Triggered, he uppercuts him and delivers a quick barrages of punches before dealing a damaging blow that knocks him several feet away.
 * Berial

When fighting against the demon himself and the ice on its head is removed you can either cut up the inside of the mouth or spin him around by his tail. When in Devil Trigger, he bursts through the back with Yamato extended. During the "Rusalka phase" Buster can be used when the ladies start to attack - it is similar to Scarecrow's one (except for that DT and normal attacks are the same). Once a Rusalka falls to the ground in exhaustion, a Buster will cause Nero to pull Bael out by the feeler. And the last, if used when Bael is frozen, green orbs could spawn.
 * Bael and Dagon

When she looks tired, Nero will stab the Red Queen into her and rev it. This Buster can be also used when Echidna is enraged and starts to fly if properly timed. Additionally, if done to her egg sac, Nero will repeatedly punch it several times.
 * Echidna

Dante has the most complex "Devil Bringer system" of all the bosses. Basically, he becomes open to Snatch or Buster when in the air or executing a move. However, there are some additions to this rule:
 * Dante
 * The most important one is that player can make him open by shooting at him: Dante starts deflecting bullets back with his own pistols, which keeps him busy.
 * Sometimes during Buster in the air Dante escapes Nero's grasp: this is immediately followed by an aerial combo unless the player won't Snatch him again. More than that, Dante can get loose several times in a row.
 * If Dante is caught in Devil Trigger explosion while executing a move, he becomes open. This is especially useful if Nero is caught in one of his combos: Devil Triggering at this moment will end combo, deal some damage and leave Dante defenseless for a moment.
 * Second explosion from Charged Shot 3 will make him open anytime. It is hard to land a shot on him though.

All the other time Dante has a habit of evading Nero's long arms with rolling and then counterattacking, often with Stinger or Coyote-A.

Even if you manage to use a Buster on him, Dante still has a chance of escaping. Usually, if Buster is used when Nero is on the ground, a small sword-fighting scene will occur, and it can either end in Nero's victory or in a draw. The former results in Nero slashing Dante for damage equal or even higher than normal Buster's, and in the latter case whoever recollects first will take the advantage. You can raise your chances of a more acceptable result by mashing Buster button during the sequence. However, in some conditions the sequence doesn't occur at all: if Nero Bustered him in the air, if Dante was using the Royalguard style at the moment or if Dante was Bustered after being dragged out of a combo by damage (melee or DT-explosion), it will give a simple Scarecrow animation.

After his sword slashes, Nero will grab his shield and punch it, reducing its aura. When used after the shield stops glowing, he punches him viciously four times before delivering a deadly uppercut to him. In Devil Trigger he still punches him four times but this time throws Credo into the air. He then jumps up and stabs Red Queen into Credo while the Devil Spirit stabs Yamato into him. Then they rip the swords out, and send Credo rolling. When he throws his lance, Nero will grab it and throw it back, impaling him, though he must be locked-on to aim correctly. Also if use Snatch instead of Buster, lance will be thrown away and small green orb will spawn.
 * Angelo Credo

Nero throws Agnus into air, then slams him back into ground and stabs him with two Gladii, then kicks him away. While Devil Triggered, Nero uses more Gladii as well as the Red Queen and Yamato.
 * Angelo Agnus

After his barriers are destroyed, Nero will throw a barrage of punches, berates him while holding Sanctus by his collar, and then finish with a powerful uppercut. When Sanctus is in the Savior it is possible to pull him out if Buster properly timed on Savior's slamming arm. After that he will be completely defenseless, which can be used for another Buster.
 * Sanctus

After his barriers are destroyed, Nero will power up a massive punch with the Devil Bringer, which slams Sanctus into a wall. When he performs a Stinger with the Sparda, Nero will grab him and perform a similar Buster as before. While Devil Triggered, the Yamato devil will deliver the blow. It is also possible to cancel out his Sparda thrusting attack and transition to this animation if timed correctly. If Sanctus Diabolica is finished off with a Buster, Nero will sometimes yell "JACKPOT!" upon impact. The moment he is finished off in this matter is repeated twice for dramatic effect.
 * Sanctus Diabolica

A Buster or Devil Buster will counter the False Savior's punches. Unlike other battles, Buster is required to defeat the boss.
 * False Savior

Background
A Rusalka is a type of Slavic ghost which tries to seduce men in order to trick them into drowning.

In Greek mythology, the Aegis was Athena's shield, made from Medusa's head, and was provided nearly perfect defense.

In Kabbalah, the Sephirot are the fruit of the "Tree of Life", which represents the ten aspects of God's manifestation in the material world.

Trivia

 * After Nero obtains the Devil Bringer, he often hides it in a sling, probably because he is ashamed of it and scared of being exiled by the townspeople.


 * Dante was surprised when Nero reveals his Devil Bringer. Stating that "You've got a trick up your sleeve..."


 * After unleashing the Devil Bringer, Nero's Blue Rose isn't shown to be reloaded anymore. Furthermore, the Red Queen's propellant system doesn't go empty, assuming that his weapons were affectec by his powers, just like Vergil and Dante.